An exhaust purification system for an internal combustion engine is known in the art, where an exhaust turbocharger is provided for driving a compressor arranged in an engine intake passage upstream of a throttle valve using an exhaust turbine arranged inside an engine exhaust passage, the engine exhaust passage upstream of the exhaust turbine and the engine intake passage downstream of the throttle valve are connected by a high pressure exhaust gas recirculation passage while a high pressure exhaust gas recirculation control valve is arranged in the high pressure exhaust gas recirculation passage, the engine exhaust passage downstream of the exhaust turbine and the engine intake passage upstream of the compressor are connected by a low pressure exhaust gas recirculation passage while a low pressure exhaust gas recirculation control valve is arranged in the low pressure exhaust gas recirculation passage, and rich control is performed to hold an air-fuel ratio of an exhaust gas discharged from an engine combustion chamber temporarily richer than the stoichiometric air-fuel ratio by injecting additional fuel into a cylinder in an expansion stroke or exhaust stroke while an exhaust gas recirculation rate is made lower than a base exhaust gas recirculation rate. In this exhaust purification system, when starting rich control, the exhaust gas recirculation rate is lowered by decreasing an amount of the exhaust recirculation gas from the high pressure exhaust gas recirculation passage, while when ending the rich control, the exhaust gas recirculation rate is reset by resetting the amount of the exhaust recirculation gas from the high pressure exhaust gas recirculation passage.
In this regard, when rich control is being performed, a temperature of the exhaust gas discharged from a combustion chamber is considerably high. For this reason, when ending rich control, if immediately resetting the amount of the exhaust recirculation gas from the high pressure exhaust gas recirculation passage, a large amount of high temperature exhaust gas will flow into the high pressure exhaust gas recirculation passage and the high pressure exhaust gas recirculation passage or high pressure exhaust gas recirculation control valve will be liable to be damaged by the heat.
In this regard, it might seem that if continuing to decrease the amount of the exhaust recirculation gas from the high pressure exhaust gas recirculation passage for a certain time after the rich control was performed, the problem of damage due to heat could be solved. However, if the exhaust gas recirculation rate is held low, production of NOX might not be able to be reduced.
Therefore, an exhaust purification system of an internal combustion engine is known in the art which, when ending rich control, the injection of additional fuel is stopped and an amount of exhaust recirculation gas from the low pressure exhaust gas recirculation passage is increased and held while reducing the amount of the exhaust recirculation gas from the high pressure exhaust gas recirculation passage, then, when a delay time elapses, the amount of exhaust recirculation gas from the high pressure exhaust gas recirculation passage and the amount of exhaust recirculation gas from the low pressure exhaust gas recirculation passage are reset (see PLT 1). That is, up until the elapse of the delay time from the end of rich control, the exhaust recirculation gas from the high pressure exhaust gas recirculation passage continues to be decreased. Further, a temperature of the exhaust recirculation gas supplied from the low pressure exhaust gas recirculation passage is relatively low. Therefore, a danger of damage due to heat is reduced. At the same time, the exhaust recirculation gas from the low pressure exhaust gas recirculation passage is increased, so the production of NOX can be reliably suppressed.